1. Field of the Invention
The present invention relates generally to injection molding machines and, more particularly, to an injection molding machine that uses a single electric motor to rotate the feed screw and, alternatively, drive a hydraulic pump stack.
2. Description of the Related Art
The injection unit of a typical injection molding machine provides essentially two functions during the course of a normal cycle of operation; namely, injection and extruder. The injection function occurs when the feed screw is moved forward linearly (without rotation) to force plastic melt into the mold. The extruder function is accomplished when the feed screw is rotated to plasticize additional material for the next shot. As the feed screw is rotated during the extruder function, the plastic melt is gradually forced past the end of the screw, creating a pressure or force to move the screw rearward to its pre-injection position as the melt accumulates.
Both the injection and extruder functions require an associated drive apparatus in the injection unit. In prior art hydraulic machines, the movement for the injection function is typically performed by a hydraulic cylinder, while the rotation of the feed screw for extruder run is normally accomplished by a hydraulic motor. More recently, electric motors combined with mechanical systems have been used as the direct power source in the injection unit. Some of the prior art electric systems have used separate motors for each function; i.e., one motor for rotating the feed screw and a second motor in combination with a mechanism, such as a ball screw, to convert rotary motion into the linear movement required for injection. Other prior art "hybrid" machines have used an electric motor to rotate the feed screw with the remaining functions of the machine being hydraulically driven, with power provided by an electric motor diving one or more hydraulic pumps.
While the "hybrid" machine incorporates some of the advantages of both electric (better control of screw rotation) and hydraulic (lower overall cost) machines, there remains room for improvement. In particular, there is potential for a more economical system since there is excess capacity in the electric motor that rotates the screw. This motor is only used during the portion of the cycle were the thermoplastic material is extruded (plasticated) to build the shot. Since the motor and the associated variable speed drive have a relatively high cost, it is desirable to maximize the utilization of this motor. Furthermore, for the injection molding machines with variable speed motors currently available, the motors are either dedicated to specific axes (as with electromechanical systems), or are applied to standard hydraulic circuits redundantly so that no economy of control is gained by the variable speed motor and drive.